change maxlength of input text field onblur event

change maxlength of input text field onblur event

instance= i need to accept maximum 5 digit value(12345) in my input field
and convert to float variable with fixed 3. (12.345) conversion is done by
calling javascript method convert();. But now the field value size is 6
including "."(dot). So how to change the maxlenth of this input field
ONBLUR event.
plz help me in sorting it out. Thank you.